docs: def add_raw_data(self, data, attrs):
          """ Add raw data for a read.
  
          :param data: The raw data DAQ values (16 bit integers).
  
          The read must already exist in the file. It must not already
          have raw data.
          """
          self.assert_writeable()
          if "Raw" not in self.handle:
              self.handle.create_group("Raw")
          if "Signal" in self.handle['Raw']:
              msg = "Fast5 file already has raw data for read '{}' in: {}"
              raise KeyError(msg.format(self.read_id, self.filename))
          self.handle['Raw'].create_dataset('Signal', data=data, compression='gzip', shuffle=True, dtype='i2')
          self._add_attributes("Raw", attrs, clear=True)